Flame ionization detection burner assemblies for use in compressible fluid-based chromatography systems

ABSTRACT

The present disclosure relates to burner assemblies of flame-based detectors. These burner assemblies are configured to deliver decompressed mobile phase of supercritical fluid chromatography systems to the flame of a flame-based detector while providing for improved optimization of analyte response as well as enhanced flame stability during operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/713,439, filed May 15, 2015, which claims the benefit of and priorityto U.S. Provisional Application No. 61/994,353, filed May 16, 2014,which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to flame ionization detection burnerassemblies and their use in compressible fluid-based chromatographysystems.

BACKGROUND

Flame ionization detection (FID) was originally developed for use inconjunction with gas chromatography and is nominally designed to operateat relatively low mobile phase flow rates (i.e., up to around 40mL/min). However, FID is now also commonly employed in conjunction withcompressible fluid-based chromatography, hereinafter referred to as“CFC,” in which the mobile phase, typically compressed carbon dioxide,is used. An example of CFC is supercritical fluid chromatography,hereinafter referred to as “SFC,” in which carbon dioxide in itssupercritical state or near supercritical state is typically used as themobile phase. When decompressed, the CFC mobile phase achieves muchhigher flow rates than that of gas chromatography. When interfacing aCFC system to an FID detector, a transfer line connected to the CFCsystem transports some or all of the mobile phase flow to the detector.Due to the compressed nature of the mobile phase, this transfer linemust also function as a flow restrictor (i.e., to maintain systempressure). The compressed mobile phase enters the restrictor as a densefluid and exits as a decompressed gas. Since the fluid expands as ittransitions to a gas, the volumetric flow rate at the outlet of therestrictor is considerable. As a result of this expansion, precisepositioning of the end of the restrictor within the FID burner isrequired to ensure stable flame operation and optimal analyte response.

Conventional restrictors in FID burner assemblies are configured suchthat the flow stream exits the restrictor and into the burner in adirection substantially parallel to the longitudinal axis of the burner.This configuration can be accomplished by simply cutting the end of therestrictor at an angle perpendicular to its longitudinal axis (i.e., a“square cut” restrictor) and is done for ease of manufacture and toensure restrictor-to-restrictor reproducibility. However, thisrestrictor design requires considerable burner length to allow for themobile phase to fully decompress and slow in linear velocity so as toboth maintain a stable flame and achieve an optimal analyte response.Thus, the use of “square cut” restrictors in FID burners results in anarrow window of distance from the flame in which the position of therestrictor must be precisely optimized. In a worst case scenario, themobile phase flow rate out of the restrictor may be so great that theFID burner may not be long enough for optimal positioning of therestrictor at all. A further complication encountered when using suchrestrictors is that its position within the FID burner must bere-optimized when any change in restrictor flow rate is experienced,such as when the system pressure is changed (i.e., density programmedseparations) while operating a split-flow interface to the FID or whenthe mobile phase flow rate is changed while employing a full-flowinterface (i.e., changing column diameter).

Thus, there exists a need for improved FID burner assemblies that do notrequire precise positioning and re-positioning of the restrictor inorder to optimize analyte response and which provide for enhanced flamestability during operation.

SUMMARY OF THE INVENTION

The present disclosure relates to FID burner assemblies and their use inCFC systems. In general and according to certain embodiments, FID burnerassemblies of the present disclosure are configured to deliver adecompressed mobile phase fluid (e.g., CO₂) from a CFC system to theflame within the detector while providing for improved optimization ofanalyte response as well as enhanced flame stability during operation.

In one embodiment, the present disclosure relates to a burner assemblyof a flame-based detector. The burner assembly comprises a burner and arestrictor. The burner comprises a burner body having a fluid inlet forreceiving combustion gases and a fluid outlet for delivering combustiongases to a flame position. The burner body defines a flow path extendingfrom the fluid inlet to the flame position and having a longitudinalaxis. The restrictor comprises a hollow body comprising a first end forreceiving at least a portion of a mobile phase flow stream from achromatography system and a second end for delivering at least a portionof the mobile phase flow stream as a decompressed mobile phase flowstream to the burner. During flame-based detection of one or moreconstituents of the at least a portion of the mobile phase flow stream(1) at least the second end of the restrictor is inserted into theburner and (2) the second end of the restrictor is adapted to deliverthe decompressed mobile phase flow stream to the burner body flow pathat an angle substantially non-parallel to the longitudinal axis of theburner.

In another embodiment, the present disclosure relates to a burnerassembly of a flame-based detector. The burner assembly comprises aburner and a restrictor. The burner comprises a burner body having afluid inlet for receiving combustion gases and a fluid outlet fordelivering combustion gases to a flame position. The burner body definesa flow path extending from the fluid inlet to the flame position andhaving a burner longitudinal axis. The restrictor comprises a hollowbody comprising a first end for receiving at least a portion of a mobilephase flow stream from a chromatography system and a second end fordelivering the at least a portion of the mobile phase flow stream as adecompressed mobile phase flow stream to the burner. The restrictor hasa restrictor longitudinal axis. During flame-based detection of one ormore constituents of the at least a portion of the mobile phase flowstream (1) at least the second end of the restrictor is located withinthe burner and (2) the restrictor longitudinal axis is substantiallynon-parallel to the burner longitudinal axis.

In another embodiment, the present disclosure is directed to a burnerassembly of a flame-based detector. The burner assembly comprises aburner and a restrictor. The burner comprises a burner body having afluid inlet for receiving combustion gases and a fluid outlet fordelivering combustion gases to a flame position. The burner body definesa flow path extending from the fluid inlet to the flame position andhaving a longitudinal axis. The restrictor comprises a hollow bodyhaving a first end for receiving at least a portion of a mobile phaseflow stream from a chromatography system and a second end for deliveringthe at least a portion of the mobile phase flow stream as a decompressedmobile phase flow stream to the burner. During flame-based detection ofone or more constituents of the at least portion of the mobile phaseflow stream (1) at least the second end of the restrictor is positionedwithin the burner; and (2) the burner is adapted so that the at least aportion of the decompressed mobile phase flow stream travels through theflow path in one or more directions substantially non-parallel to thelongitudinal axis.

In another embodiment, the present disclosure is directed to a burnerassembly of a flame-based detector. The burner assembly comprises aburner and a restrictor. The burner comprises a burner body having afluid inlet for receiving combustion gases and a fluid outlet fordelivering combustion gases to a flame position. The burner body has alongitudinal axis and further comprises an interior wall surfacedefining an inner perimeter of the burner body and one or more membersextending from the interior wall surface at an angle substantiallynon-parallel to the longitudinal axis. The restrictor comprises a hollowbody having a first end for receiving at least a portion of a mobilephase flow stream from a chromatography system and a second end fordelivering the at least a portion of the mobile phase flow stream as adecompressed mobile phase flow stream to the burner. During flame-baseddetection of one or more constituents of the at least portion of themobile phase flow stream (1) at least the second end of the restrictoris contained within the burner and (2) the one or more members extendingfrom the interior wall are dimensioned and configured to deflect thedecompressed mobile phase flow stream in a direction substantiallynon-parallel to the longitudinal axis.

In another embodiment, the present disclosure is directed to a method ofmaintaining a flame in a burner assembly of a flame-based detector. Themethod comprises at least three steps. The first step of the methodcomprises providing the burner assembly. The burner assembly comprises aburner and a restrictor. The burner comprises a burner body having afluid inlet for receiving combustion gases and a fluid outlet fordelivering combustion gases to a flame position having a flame. Theburner body defines a flow path extending from the fluid inlet to theflame position and having a burner longitudinal axis. The restrictorcomprises a restrictor comprising a hollow body comprising a first endfor receiving at least a portion of a mobile phase flow stream from achromatography system and a second end for delivering the at least aportion of the mobile phase flow stream as a decompressed mobile phaseflow stream to the burner. The second end of the restrictor is sized andinserted into the inner burner. The second step of the method comprisespassing at least a portion of the mobile phase flow stream through therestrictor at a decompressed flow rate of 40 mL/min or greater. Thethird step of the method comprises delivering at least the portion ofthe mobile phase flow stream into the burner and to the flame positionas the decompressed mobile phase flow stream at a force/velocityinsufficient to extinguish the flame.

In another embodiment, the present disclosure is directed to a method ofmaintaining a flame in a burner assembly of a flame-based detector. Themethod comprises at least three steps. The first step of the methodcomprises providing the burner assembly. The burner assembly comprises aburner and a restrictor. The burner comprises a burner body having afluid inlet for receiving combustion gases and a fluid outlet fordelivering combustion gases to a flame position having a flame. Therestrictor comprises a restrictor comprising a hollow body comprising afirst end for receiving at least a portion of a mobile phase flow streamfrom a chromatography system and a second end for delivering the atleast a portion of the mobile phase flow stream as a decompressed mobilephase flow stream to the burner. The second end of the restrictor sizedand inserted into the burner. The second step of the method comprisespassing at least the portion of the mobile phase flow stream through therestrictor. The third step of the method comprises delivering at least aportion of the mobile phase flow stream into the burner and to the flameposition as the decompressed mobile phase flow stream such that thedecompressed mobile phase flow stream flows to the flame along anon-parallel fluid flow path.

In another embodiment, the present disclosure is directed to a method ofmaintaining a flame in a burner assembly of a flame-based detector. Themethod comprises at least three steps. The first step of the methodcomprises providing the burner assembly. The burner assembly comprises aburner and a restrictor. The burner comprises a burner body having afluid inlet for receiving combustion gases and a fluid outlet fordelivering combustion gases to a flame position having a flame. Theburner body defines a flow path extending from the fluid inlet to theflame position and having a burner longitudinal axis. The restrictorcomprises a restrictor comprising a hollow body comprising a first endfor receiving at least a portion of a mobile phase flow stream from achromatography system and a second end for delivering the at least aportion of the mobile phase flow stream as a decompressed mobile phaseflow stream to the burner. The second end of the restrictor is sized andinserted into the inner burner. The second step of the method comprisespassing at least a portion of the mobile phase flow stream through therestrictor. The third step of the method comprises delivering at leastthe portion of the mobile phase flow stream into the burner and to theflame position as the decompressed mobile phase flow stream such that astable flame is maintained and optimal analyte response is achievedregardless of the distance of the restrictor second end from the flameposition.

The above embodiments can include one or more of the following features.In some embodiment, the mobile phase flow stream can comprise carbondioxide. In some of those embodiments, the second end of the restrictorcan be adapted to deliver the decompressed mobile phase flow stream atan angle of at least 25 degrees with respect to the longitudinal axis ofthe inner burner. The second end of the restrictor in some of the aboveembodiments can also be adapted to provide radial decompression of themobile phase. Alternatively, it can also comprise a frit or a pintle. Insome embodiments, the restrictor is positioned relative to the burnersuch that the longitudinal axis of the restrictor is at a 90 degreeangle relative to that of the burner. In certain embodiments, the flowpath of the burner is tortuous or is packed with glass wool. In otherembodiments, the interior wall surface of the burner defines a tortuouspath between the second end of the restrictor and the flame.Alternatively or in addition thereto, the members extending from theinterior wall surface of these burners can be baffles, porous, and/ortapered.

The embodiments of the present disclosure provide advantages over theprior art based on their unique configurations and performanceproperties. For example, the “square cut” restrictors of conventionalFID burner assemblies require precise positioning (and re-positioningwhen system pressure is changed) of the restrictor in order to optimizeanalyte response and to maintain flame stability during operation. Incontrast, the burner assemblies of the present disclosure do not requireprecise positioning and re-positioning of the restrictor in order tooptimize analyte response and which provide for enhanced flame stabilityduring operation. For example, in certain embodiments of the presentdisclosure, optimal analyte response is achieved and flame stabilitymaintained regardless of the where the restrictor is positioned insidethe burner.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages provided by the presentdisclosure will be more fully understood from the following descriptionof exemplary embodiments when read together with the accompanyingdrawings.

FIG. 1 schematically depicts an exemplary SFC system interfaced with anFID detector, including an enlarged cross-section detail of the FIDdetector burner assembly.

FIG. 2 depicts a cross-section of a burner assembly with a conventional“square cut” restrictor.

FIG. 3 depicts a cross-section of a burner assembly according to thepresent disclosure where the restrictor has a tip cut at an angle ofless than 90°.

FIG. 4 depicts a cross-section of a portion of a burner assemblyaccording to the present disclosure where the restrictor has a drilledtip.

FIG. 5A depicts a cross-section of a portion of a burner assemblyaccording to the present disclosure where the restrictor has a frittedtip.

FIG. 5B depicts a cross-section of a portion of a burner assemblyaccording to the present disclosure where the restrictor has a frittedtip, the top of which has been sealed.

FIG. 6 depicts a cross-section of a portion of a burner assemblyaccording to the present disclosure where the restrictor has a tipcomprising a pintle.

FIG. 7 depicts a cross-section of a burner assembly according to thepresent disclosure where the restrictor is inserted into a side of theburner.

FIG. 8 depicts a cross-section of a burner assembly according to thepresent disclosure where the flow path of the burner is partiallyoccluded by glass wool or a frit.

FIG. 9 depicts a cross-section of a burner assembly according to thepresent disclosure where the flow path of the burner is partiallyoccluded by baffles.

FIG. 10A depicts a cross-section of an integral restrictor according tothe present disclosure.

FIG. 10B depicts a cross-section of a tapered restrictor according tothe present disclosure.

FIG. 10C depicts a cross-section of a converging-diverging restrictoraccording to the present disclosure.

FIG. 10D depicts a cross-section of a linear hybrid restrictor accordingto the present disclosure.

DETAILED DESCRIPTION

In various aspects, configurations, and embodiments, the presentdisclosure provides novel burner assemblies of flame-based detectors, aswell as methods of maintaining a flame in such burner assemblies.

As used herein, the phrase “chromatography system” refers to an assemblyor array of interconnected components that is used to separate a mixtureof compositions. The mixture is dissolved in a fluid mobile phase thatcarries it through a structure holding a stationary phase. The variousconstituents of the mixture travel at different speeds through thestationary phase, thereby causing them to separate. An example of such achromatography system includes, but is not limited to, a compressiblefluid-based (CFC) system. An example of a CFC system, includes, but isnot limited to, CO₂-based a supercritical fluid chromatography (SFC)system, which employs a supercritical fluid or near supercritical fluidas the mobile phase. That is, the mobile phase includes CO₂ (andpotentially other modifiers) at or near supercritical conditions of themobile phase for at least some portion of chromatographic process. Insome embodiments the CO₂ mobile phase does not actually reach thesupercritical state, but is highly compressed. The highly compressed CO₂mobile phase provides similar advantages to supercritical CO₂ for thepurposes of chromatography and is therefore considered to be nearsupercritical with respect to the performance of SFC.

As used herein, the phrase “flame-based detection” refers to thedetection energetic particles (e.g., electrons, photons, etc.) formedduring combustion of compounds in a flame. Both organic and inorganiccompounds can be ionized and detected. Examples of combustion gases usedto produce the flame include, but are not limited to, mixtures ofhydrogen and air/oxygen. Nitrogen (N₂) is commonly employed as a makeupgas in such mixtures. Examples of such flame-based detection include,but are not limited to, flame ionization detection (FID), flamephotometric detection, chemiluminescence nitrogen detection, thermionicdetection (e.g., nitrogen-phosphorus detection), and chemiluminesencesulfur detection.

As used herein, the phrase “longitudinal axis” refers to the axis ofsymmetry that runs lengthwise from the fluid inlet, through the flowpath, to the fluid outlet of the burner body or the axis of symmetrythat runs lengthwise from the first end, through the hollow body, to thesecond end of the restrictor.

As used herein, the phrase “decompressed” refers to the phase transitionof the supercritical fluid of the mobile phase from a liquid, asupercritical fluid, or a highly compressed gas to a gas.

As used herein, the phrase “substantially non-parallel” refers to theangle at which the mobile phase flow stream decompresses (i.e., the“angle of decompression”), relative to the longitudinal axis of theburner, sufficient to prevent the decompressed mobile phase flow streamfrom extinguishing the flame at the flame position. Such an angle can beany angle greater than 0° to 180°, relative to the longitudinal axis ofthe burner. Examples of such angles include, but are not limited to, 5,10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95,100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 160, 165, 170,and 175, and 180°. In some embodiments, the angle is greater than 22°.In other embodiments, the angle is 45, 67, 90, or 135°.

An example of an SFC system that employs supercritical CO₂ as the mobilephase is illustrated in FIG. 1. SFC system 100 includes CO₂ pump 110,which pumps CO₂ from CO₂ source 120 into system 100 at a pressure thatmaintains the CO₂ in a dense state, column 130, back pressure regulator(BPR) 140, and flame ionization detection (FID) detector 150. The CO₂mobile phase flows from CO₂ pump 110 to column 130. Injector 160 forinjecting mixtures to be separated is located in-line between CO₂ pump110 and column 130. Column 130 is housed in column oven 170, whichmaintains column 130 at a constant temperature. Column oven 170 alsofunctions to preheat the supercritical CO₂ mobile phase and the mixturefor separation to the constant temperature of column 130 prior to theirentry. Flow splitter 180 is located in-line between column oven 170 andBPR 140. Flow splitter 180 operates to divert at least a portion of themobile phase flow, which now contains the separated constituentcompounds of the mixture, (i.e., the column effluent), to FID detector150, where one or more compounds are ionized via combustion in burnerassembly 190 of FID 150.

As discussed above, one component of an FID detector is the burnerassembly. A cross-section of an exemplary burner assembly is illustratedin FIG. 2. Assembly 200 includes burner 210 and restrictor 220, aportion of which located inside burner 210. A portion of burner 210 is,in turn, located inside burner housing 230. Inlets 240 and 250 supplyassembly 200 with fuel gas and oxidant gas, respectively, (together,combustion gases), which mix at flame position 260 and are thencombusted. The flame produced at position 260 is a “diffusion” flame,the exterior of which is an oxidant-rich region and the interior ofwhich is a fuel gas-rich region. The second end 224 of restrictor 220can be positioned inside burner 210 at any distance 296 relative toflame position 260. Column effluent 226 is fed to the FID detector,entering restrictor 220 at its first end 222 and exiting at its secondend 224 at an angle substantially parallel to the longitudinal axis 212of burner 210, where it decompresses and travels through flow path 218of burner 210 to flame position 260, where one or more separatedconstituent compositions of the mixture are ionized via combustion. Theelectrons released during ionization are attracted to collectorelectrode 290, where they induce a current, which is, in turn, fed toelectrometer 292.

As can be seen in FIG. 2, the second end 224 of restrictor 220 is“square cut.” This geometry results in the mobile phase flow having amean direction 216 substantially parallel to the longitudinal axis 212of burner 210 as it exits the second end 224 of restrictor 220 anddecompresses into flow path 218. One problem with this restrictorgeometry is that re-positioning of the second end 224 of restrictor 220relative to flame position 260 may be required any time the flow rate ofthe mobile phase is changed in order to optimize detector response.Another problem is that, depending on the flow rate of the mobile phase,it may not be possible with this restrictor geometry to maintain astable flame at certain distances 296 (or even regardless of thedistance) of the second end 224 from flame position 260.

These problems are solved by the various burner assembly configurationsof the present disclosure.

All of the burner assembly configurations of the present disclosurecomprises a burner and a restrictor. The burners of these configurationscomprise a burner body having a fluid inlet for receiving combustiongases and a fluid outlet for delivering at least a portion of thecombustion gases to a flame position. The burner bodies define a flowpath extending from the fluid inlet to the flame position and having alongitudinal axis. The restrictors of these configurations comprise ahollow body comprising a first end for receiving at least a portion of amobile phase flow stream from a chromatography system and a second endfor delivering the at least a portion of the mobile phase flow stream asa decompressed mobile phase flow stream to the burner. In allconfigurations, at least the second end of the restrictor is insertedinto the burner during flame-based detection of one or more constituentsof the at least a portion of the mobile phase flow stream.

During flame-based detection, the flame-based detector is typicallymaintained at a temperature above ambient temperature. For example, theflame-based detector can be maintained at a temperature in the range offrom about 50 to 500° C. or higher. In certain embodiments, theflame-based detector can be held at a temperature of about 50, 60, 70,80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220,230, 240, 250, 260, 270, 280, 290, 300, 310, 320, 330, 340, 350, 360,370, 380, 390, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, or 500°C. during flame-based detection.

Both the burner and the restrictor of the burner assemblies of thepresent invention can be fabricated from any material capable ofwithstanding the temperatures at which the flame-based detectors aremaintained. These materials also should not “de-gas” or otherwiseintroduce extraneous carbon containing compounds into the FID flame.Such materials include, but are not limited to metals, ceramics, glass,or polymers. In one embodiment, the burner is fabricated from a metal,such as steel. In one embodiment, the restrictor is fabricated fromglass. Furthermore, the restrictor and its second end can be fabricatedas separate, optionally disposable/replaceable, pieces that can befitted together. These two pieces can be fabricated from the same ordifferent materials.

The cross-sectional dimensions of the burner and the restrictor can beany width and shape, with the only conditions being that the width andshape of the restrictor should be such that it can be inserted into theburner and there is space between the inner surface of the burner andthe outer surface of the restrictor sufficient to allow combustiongas(es) to move freely towards the flame position. In certainembodiments, the cross-sectional width of the burner can be about 15,14, 13, 12, 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0.762, 0.75, 0.5, 0.457,0.28, or 0.25 mm. In certain embodiments, the cross-sectional width ofthe restrictor can be about 2.0, 1.9, 1.8, 1.7, 1.6, 1.5, 1.4, 1.3,1.2., 1.1, 1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.2, 0.1, 0.090,0.080, 0.070, 0.060, 0.050, 0.049, 0.048, 0.047, 0.046, 0.045, 0.044,0.043, 0.042, 0.041, 0.040, 0.039, 0.038, 0.037, 0.036, 0.035, 0.034,0.033, 0.032, 0.031, 0.030, 0.029, 0.028, 0.027, 0.026, 0.025, 0.024,0.023, 0.022, 0.021, 0.020, 0.010, 0.009, 0.008, 0.007, 0.006, 0.005,0.004, 0.003, 0.002, or 0.001 mm. In certain embodiments, thecross-sectional shape of the burner and/or the restrictor can be acircle, an oval, a square, a rectangle, or a triangle. In a microfluidicembodiment, the cross-sectional shape of the burner can be trapezoidalor “gumdrop” shaped.

The longitudinal dimensions of the burner and the restrictor can be anylength and shape. In certain embodiments, the length of the burner canbe about 45, 44, 43, 42, 41, 40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30,29, 28, 27, 26, 25, 24, 23, 22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12,11, 10, 9, 8, 7, 6, 5, 4, 3, 2, or 1 mm. In certain embodiments, thelength of the restrictor can be about 200, 199, 198, 197, 196, 195, 194,193, 192, 191, 190, 189, 188, 187, 186, 185, 184, 183, 182, 181, 180,179, 178, 177, 176, 175, 174, 173, 172, 171, 170, 169, 168, 167, 166,165, 164, 163, 162, 161, 160, 159, 158, 157, 156, 155, 154, 153, 152,151, 150, 149, 148, 147, 146, 145, 144, 143, 142, 141, 140, 139, 138,137, 136, 135, 134, 133, 132, 131, 130, 129, 128, 127, 126, 125, 124,123, 122, 121, 120, 119, 118, 117, 116, 115, 114, 113, 112, 111, 110,109, 108, 107, 106, 105, 104, 103, 102, 101, 100, 99, 98, 97, 96, 95,94, 93, 92, 91, 90, 89, 88, 87, 86, 85, 84, 83, 82, 81, 80, 79, 78, 77,76, 75, 74, 73, 72, 71, 70, 69, 68, 67, 66, 65, 64, 63, 62, 61, 60, 59,58, 57, 56, 55, 54, 53, 52, 51, 50, 49, 48, 47, 46, 45, 44, 43, 42, 41,40, 39, 38, 37, 36, 35, 34, 33, 32, 31, 30, 29, 28, 27, 26, 25, 24, 23,22, 21, 20, 19, 18, 17, 16, 15, 14, 13, 12, 11, or 10 cm. In certainembodiments, the longitudinal shape of the burner and/or restrictor canbe cylindrical or tubular. In other embodiments, the longitudinal shapeof the burner and/or restrictor can be cylindrical or tubular, pinchedin one more positions along the length of the burner and/or restrictor,or undulates along the length of the burner and/or restrictor, such thatthe flow of the mobile phase and/or the combustion gas(es) converge anddiverge along the flow path. In one embodiment, the restrictor can belonger than the burner. In another embodiment, the restrictor can be 35cm long and the burner can be 25 mm long.

In certain embodiments, the combustion gases can be hydrogen, as thefuel gas, in mixture with an oxidant gas, such as oxygen or air. Use ofcarbon-based gases should be avoided. Nitrogen can be used as a makeupgas. In one embodiment, the combustion gases are hydrogen and air oroxygen, with nitrogen as the makeup gas.

The mobile phase can comprise any supercritical fluid. In certainembodiments, the supercritical fluid can be CO₂, N₂, Ar, Xe, achlorofluorocarbon, a fluorocarbon, N₂O, H₂O, or SF₆. The mobile phasecan also comprise one or more FID-compatible modifiers. Example of suchmodifiers include, but are not limited to, formic acid, trifluoroaceticacid, and other highly oxidized carbon-containing compounds.

In one of the above burner assembly configurations, the second end ofthe restrictor is adapted to deliver the decompressed mobile phase flowstream to the burner body flow path at an angle substantiallynon-parallel to the longitudinal axis of the burner. This adaptation ofthe second end of the restrictor can be achieved in any number of ways.For example, the second end can be “angled,” i.e., the second end can becut at an angle less than or greater than perpendicular to thelongitudinal axis of the restrictor (90°). The “angle of cut” includesany angle in the range of from greater than 0° to less than 90° and therange of from greater than 90° to less than 180°. Such an angle of cutresults in an angle of decompression of the mobile phase flow streamthat is substantially non-parallel to the longitudinal axis of theburner. Example of such angles of cut include, but are not limited to,5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 95,100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165,170, and 175°. In some embodiments, the angle of cut results in an angleof decompression that is greater than 22°. In other embodiments, theangle of cut results in an angle of decompression that is 45, 67, 90, or135°. Other examples of such adaptations include, but are not limitedto, (1) a restrictor where the second end has been roughly cleaved,resulting in a jagged end surface, (2) a restrictor where the opening atthe second end is sealed and a second opening is formed by drilling oneor more holes into the side of the restrictor (i.e., a “drilled”restrictor), (3) a restrictor fitted with a frit (i.e., a “fritted”restrictor), wherein the top of the frit is optionally sealed, (4) arestrictor fitted with a pintle (i.e., a “pintled” restrictor) (5) anintegral restrictor, (6) a restrictor where the second end has beentapered, such that the restrictor flow path dimensions perpendicular tothe longitudinal axis of the restrictor decrease as the flow pathapproaches the second end, i.e., the flow path narrows as it approachesthe second end, (7) a converging/diverging restrictor, and (8) a linearhybrid restrictor. Each of restrictor examples (5), (6), (7), and (8)have short decompression zones and are easily plugged. Illustrations ofexemplary restrictors (5), (6), (7), and (8) are provided in FIGS. 10A,10B, 10C, and 10D, respectively. That is, FIG. 10A shows across-sectional view of an embodiment of an integral restrictor 1025;FIG. 10B shows a cross-sectional view of an embodiment of a restrictorwith a tapered second end 1026; FIG. 10C shows a cross-sectional view ofan embodiment of converging/diverging restrictor 1027; and FIG. 10Dshows a cross-sectional view of an embodiment of a linear hybridrestrictor 1028.

An example of a burner assembly configuration where the second end ofthe restrictor is angled is illustrated in FIG. 3. Burner assembly 300comprises a burner 310 and restrictor 320. Burner 310 comprises a burnerbody having fluid inlets 340 and 350 for receiving combustion gases anda fluid outlet 314 for delivering at least a portion of the combustiongases to flame position 360. Restrictor 320 comprises a first end 322and a second end 324. Second end 324 of restrictor 320 is located withinburner 310. A portion of burner 310 is, in turn, located inside burnerhousing 330. Second end 324 of restrictor 320 can be positioned insideburner 310 at any distance 396 relative to flame position 360. Columneffluent 326 is fed to the FID detector, entering restrictor 320 at itsfirst end 322 and exiting at its second end 324 at an angle 316substantially non-parallel to the longitudinal axis 312 of burner 310.The mobile phase of the column effluent decompresses and travels throughflow path 318 of burner 310 to flame position 360, where one or moreseparated constituent compositions of the mixture are ionized viacombustion. These ions are attracted to collector electrode 390, wherethey induce a current, which is, in turn, fed to electrometer 392.

An example of a burner assembly configuration having a “drilled”restrictor is illustrated in FIG. 4. In this example, the configurationof the burner assembly is identical to that of FIG. 3 except for theadaptation to the second end of the restrictor. Column effluent exits atthe second end 424 of restrictor 420 at an angle 416 substantiallynon-parallel to the longitudinal axis 412 of burner 410. The mobilephase of the column effluent decompresses and travels through flow path418 of burner 410 to the flame position. “Drilled” restrictors can befabricated from a conventional “square cut” restrictor by sealing theend of the restrictor, followed by drilling one or more holes into itsside until the interior channel of the restrictor is reached. The holecan be drilled using a laser or a mechanical drill. The holes can bedrilled at any angle in the range of from greater than 0° to less than180°, relative to the longitudinal axis of the restrictor. In oneembodiment the one or more holes are drilled at an angle of 90°,relative to the longitudinal axis of the restrictor.

Examples of a burner assembly configuration having a “fritted”restrictor is illustrated in FIGS. 5A and 5B. In these examples, theconfiguration of the burner assembly is identical to that of FIG. 3except for the adaptation to the second end of the restrictor. Columneffluent exits at the fritted second end 524 of restrictor 520 at anangle 516 substantially non-parallel to the longitudinal axis 512 ofburner 510. The tip of the fritted second end 524 can be sealed with acap 517 (FIG. 5B) to force the mobile phase to decompress radially. Themobile phase of the column effluent decompresses and travels throughflow path 518 of burner 510 to the flame position.

An example of a burner assembly configuration having a “pintle”restrictor is illustrated in FIG. 6. In this example, the configurationof the burner assembly is identical to that of FIG. 3 except for theadaptation to the second end of the restrictor. Column effluent exits atthe second end 624 of restrictor 620 and is redirected by pintle 628 atan angle 616 substantially non-parallel to the longitudinal axis 612 ofburner 610. The mobile phase of the column effluent decompresses andtravels through flow path 618 of burner 610 to the flame position.

In another of the above burner assembly configurations, the second endof the restrictor, which has a longitudinal axis, is inserted into theburner such that the restrictor longitudinal axis is substantiallynon-parallel to the burner longitudinal axis. The restrictor used insuch configurations can be a conventional “square cut” restrictor or oneof the adapted restrictors described above. In one embodiment, therespective longitudinal axes of the restrictor and the burner areperpendicular. An example of such a burner assembly configuration isillustrated in FIG. 7. The burner assembly comprises a burner 710 andrestrictor 720. Longitudinal axis 729 of restrictor 720 is perpendicularto longitudinal axis 712 of burner 710. Burner 710 comprises a burnerbody having fluid inlets 740 and 750 for receiving combustion gases anda fluid outlet 714 for delivering at least a portion of the combustiongases to flame position 760. Restrictor 720 comprises a first end 722and a second end 724. Second end 724 of restrictor 720 is located withinburner 710. A portion of burner 710 is, in turn, located inside burnerhousing 730. Column effluent 726 is fed to the FID detector, enteringrestrictor 720 at its first end 722 and exiting at its second end 724 atan angle 716 substantially non-parallel to the longitudinal axis 712 ofburner 720. The mobile phase of the column effluent decompresses andtravels through flow path 718 of burner 710 to flame position 760, whereone or more separated constituent compositions of the mixture areionized via combustion. These ions are attracted to collector electrode790, where they induce a current, which is, in turn, fed to electrometer792.

In another of the above burner assembly configurations, the burner isadapted so that the at least a portion of the decompressed mobile phaseflow stream travels through the flow path in one or more directionssubstantially non-parallel to the longitudinal axis. This can achievedby using a burner having a tortuous flow path. In one aspect, thistortuous flow path can be formed by packing the flow path of the burnerwith glass wool. In another aspect, the tortuous flow path can be formedin a burner having one or more members extending from its interior wallsurface at an angle substantially non-parallel to its longitudinal axisand which are dimensioned and configured to deflect decompressed mobilephase flow stream in a direction substantially non-parallel to itslongitudinal axis. Such members can take the form of a frit or one ormore baffles extending from the interior wall of the burner. The memberscan be porous and/or tapered. The restrictor used in such configurationscan be a conventional “square cut” restrictor or one of the adaptedrestrictors described above.

An example of such a burner assembly configuration is illustrated inFIG. 8. The burner assembly 800 comprises a burner 810 and restrictor820. Burner 810 comprises a burner body having fluid inlets 840 and 850for receiving combustion gases and a fluid outlet 819 for delivering atleast a portion of the combustion gases to flame position 860.Restrictor 820 comprises a first end 822 and a second end 824. Secondend 824 of restrictor 820 is located within burner 810. A portion ofburner 810 is, in turn, located inside burner housing 830. Columneffluent 826 is fed to the FID detector, entering restrictor 820 at itsfirst end 822 and exiting at its second end 824 at an angle 816 parallelto the longitudinal axis 812 of burner 810. The mobile phase of thecolumn effluent decompresses and travels through flow path 818 of burner810 through frit 814. As a result of passing through frit 814, thedecompressed mobile phase travels on to flame position 860 at an anglesubstantially non-parallel to the longitudinal axis 812 of burner 810,where one or more separated constituent compositions of the mixture areionized via combustion. These ions are attracted to collector electrode890, where they induce a current, which is, in turn, fed to electrometer892.

Another example of such a burner assembly configuration is illustratedin FIG. 9. The burner assembly 900 comprises a burner 910 and restrictor920. Burner 910 comprises a burner body having fluid inlets 940 and 950for receiving combustion gases and a fluid outlet 919 for delivering atleast a portion of the combustion gases to flame position 960.Restrictor 920 comprises a first end 922 and a second end 924. Secondend 924 of restrictor 920 is located within burner 910. A portion ofburner 910 is, in turn, located inside burner housing 930. Columneffluent 926 is fed to the FID detector, entering restrictor 920 at itsfirst end 922 and exiting at its second end 924 at an angle 916 parallelto the longitudinal axis 912 of burner 910. The mobile phase of thecolumn effluent decompresses and travels through flow path 918 of burner910 past baffles 914. As a result of passing by baffles 914, thedecompressed mobile phase travels on to flame position 960 at an anglesubstantially non-parallel to the longitudinal axis 912 of burner 910,where one or more separated constituent compositions of the mixture areionized via combustion. These ions are attracted to collector electrode990, where they induce a current, which is, in turn, fed to electrometer992.

The present disclosure is also directed to methods of maintaining aflame in a burner assembly of a flame-based detector. These methodscomprises the following three steps:

-   -   1) providing a burner assembly;    -   2) passing at least the portion of the mobile phase flow stream        through the restrictor at a flow rate of 40 mL/min or greater;        and either    -   3) delivering at least the portion of the mobile phase flow        stream into the burner and to the flame position as the        decompressed mobile phase flow stream at a force/velocity        insufficient to extinguish the flame; or    -   3′) delivering at least the portion of the mobile phase flow        stream into the burner and to the flame position as the        decompressed mobile phase flow stream such that the decompressed        mobile phase flow stream flows to the flame along a non-parallel        fluid flow path.        The burner assembly comprises a burner and a restrictor. The        burner comprises a burner body having a fluid inlet for        receiving combustion gases and a fluid outlet for delivering at        least a portion of the combustion gases to a flame position        having a flame. The restrictor comprises a restrictor comprising        a hollow body comprising a first end for receiving at least a        portion of a mobile phase flow stream from a chromatographic        system and a second end for delivering the at least a portion of        the mobile phase flow stream as a decompressed mobile phase flow        stream to the burner. The second end of the restrictor is sized        and inserted into the inner burner. The effects of steps 3) and        3′) can achieved by employing any of the above-described burner        assembly configurations as the burner assemblies of these        methods.

The present invention is further defined in the following Examples. Itshould be understood that these Examples, while indicating preferredembodiments of the invention, are given by way of illustration only.From the above discussion and these Examples, one skilled in the art canascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various uses andconditions.

Examples

The analytical chromatographic system used was a carbon-dioxide basedsystem (UPC²® System) commercially available from Waters TechnologiesCorporation, Milford, Mass., USA. The system included an autosampler, acolumn oven, a 3.0×100 mm ACQUITY UPC² 1.8 μm HSS C18 SB chromatographiccolumn commercially available from Waters Technologies Corporation,Milford, Mass., USA, and an automated back pressure regulator. Themobile phase was 100% carbon dioxide supplied to the system via a fluiddelivery module and was maintained at a pressure of 138 bar. The columnwas heated to a temperature of 45° C. The flow rate was 1.5 mL/min. Thesample injection volume was 0.5 μL. At the outlet of the column, andupstream of the backpressure regulator, a “T” fitting directed a portionof the mobile phase flow to the BPR and a portion of the mobile phaseflow to a FID (SRI Model 110 FID commercially available from SRIInstruments, Torrance, Calif., USA). The output signal of the FID wasanalyzed using Empower® 3 Chromatography Data Software commerciallyavailable from Waters Technologies Corporation, Milford, Mass., USA.

The measured, decompressed CO₂ flow rate directed to the FID was 100mL/min. At this CO₂ flow rate, optimal response was achieved at hydrogenand air flow rates of 97 and 800 mL/min, respectively. The FID body washeld at 350° C. The signal to noise ratio of an analyte peak wasevaluated over a range of restrictor positions within the FID. Arestrictor with a second end angle of decompression of 0 degrees (i.e.,“square cut”) provided an optimal response when positioned about 15 mmfrom the tip of the burner. Positions further from or nearer to theburner tip resulted in a decrease in signal to noise. However, when arestrictor with a tip angle of 45 degrees was employed, optimal signalto noise was achieved over the entire range of restrictor positionadjustment (i.e., response was independent of restrictor position).

TABLE 1 FID conditions Outlet flow (mL/min) Hydrogen Flow (mL/min) AirFlow (mL/min) 40 57 560 100 97 800 200 202 1040

TABLE 2 Results Table Flow Rate Decompression angle (°) (mL/min) 0 22 4567 90 135 40 X X ◯ ◯ ◯ ◯ 100 X X ◯ ? N/A N/A 200 X X ◯ X N/A N/A X -response dependent on position ? - response improved but not completelyindependent of position ◯ - response independent of position N/A - nodata

1. A burner assembly of a flame-based detector comprising: (1) a burnercomprising a burner body having a fluid inlet for receiving combustiongases and a fluid outlet for delivering combustion gases to a flameposition, the burner body defining a flow path extending from the fluidinlet to the flame position and having a longitudinal axis; and (2) arestrictor comprising a hollow body comprising a first end for receivingat least a portion of a mobile phase flow stream from a chromatographysystem and a second end for delivering the at least a portion of themobile phase flow stream after pressure reduction to the burner; andwherein during flame-based detection of one or more constituents of theat least a portion of the mobile phase flow stream: at least the secondend of the restrictor is inserted into the burner; and the second end ofthe restrictor that is angled to deliver the mobile phase flow stream tothe burner body flow path at an angle substantially non-parallel to thelongitudinal axis of the burner.
 2. (canceled)
 3. The burner assembly ofclaim 1, wherein the second end of the restrictor is adapted to deliverthe mobile phase flow stream at an angle of at least 25 degrees withrespect to the longitudinal axis of the burner. 4-11. (canceled)
 12. Aburner assembly of a flame-based detector comprising: (1) a burnercomprising a burner body having a fluid inlet for receiving combustiongases and a fluid outlet for delivering combustion gases to a flameposition, the burner body having a longitudinal axis and furthercomprising an interior wall surface defining an inner perimeter of theburner body and one or more members extending from the interior wallsurface at an angle substantially non-parallel to the longitudinal axis;and (2) a restrictor comprising a hollow body comprising a first end forreceiving at least a portion of a mobile phase flow stream from achromatography system and a second end for delivering the at least aportion of the mobile phase flow stream after pressure reduction to theburner; wherein during flame-based detection of one or more constituentsof the at least portion of the mobile phase flow stream: at least thesecond end of the restrictor is contained within the burner; and the oneor more members extending from the interior wall are dimensioned andconfigured to deflect the mobile phase flow stream in a directionsubstantially non-parallel to the longitudinal axis.
 13. The burnerassembly of claim 12, wherein the interior wall surface of the burnerdefines a tortuous path between the second end of the restrictor and theflame position.
 14. The burner assembly of claim 12, wherein the one ormore members is a baffle.
 15. The burner assembly of claim 12, whereinthe one or more members is porous.
 16. The burner assembly of claim 12,wherein the one or more members are tapered.
 17. A method of maintaininga flame in a burner assembly of a flame-based detector comprising (1)providing the burner assembly comprising: (a) a burner comprising aburner body having a fluid inlet for receiving combustion gases and afluid outlet for delivering combustion gases to a flame position havinga flame, the burner body having a longitudinal axis and furthercomprising an interior wall surface defining an inner perimeter of theburner body a and one or more member extending from the interior wallsurface at an angle substantially non-parallel to the longitudinal axis,and (b) a restrictor comprising a hollow body comprising a first end forreceiving at least a portion of a mobile phase flow stream from achromatography system and a second end for delivering the at least aportion of the mobile phase flow stream after pressure reduction to theburner, the second end of the restrictor sized and inserted into theinner burner; (2) passing at least a portion of the mobile phase flowstream through the restrictor; and (3) delivering at least the portionof the mobile phase flow stream into the burner and to the flameposition such that the mobile phase flow stream flows to the flame alongone or more fluid paths non-parallel to the longitudinal axis. 18-21.(canceled)
 22. The method of claim 17, wherein the at least a portion ofthe mobile phase flow stream is passed through the restrictor at a flowrate of 40 mL/min or greater.